1. Field of the Invention
This invention relates to an aqueous dispersion of organic pigment suitable for coloring building materials such as concrete, asphalt, plaster, mortar and cement mortar. More particularly, this invention relates to aqueous dispersions which contain silica binder, an organic pigment and dispersing agent.
2. Description of the Prior Art
It is desirable to color exposed concrete surfaces for both aesthetic and functional reasons. Colored concrete buildings do not present an environment as sterile and cold as white concrete. In addition, light colors can be used in sunny climates to help reduce glare while darker colors may be used to increase a building""s heat storage capacity in cooler climates.
Coloring the exposed surface by painting or by coating the surface with some other decorative layer is known. U.S. Pat. Nos. 2,549,516; 3,929,692 and 4,1311,956 disclose compositions for covering exposed concrete surfaces. However, painting or application of a coating layer is an additional step in construction which adds cost and complexity to a construction project. Moreover, an exterior painted surface may require repainting in a relatively short period of time.
Another approach is to add a color additive to the building material, thereby eliminating the extra step associated with painting the building surface. However, any color additive must be uniformly dispersed throughout the building material. This can be difficult given the low intensity mixing and short mixing times customary in the building materials industry. Moreover, the additive must not adversely affect the desirable properties of the building material such as the strength or setting behavior of concrete or reduce the compressive strength or abrasion resistance of asphalt. See ASTM C 979-82 xe2x80x9cStandard Specification for Pigments for Integrally Colored Concrete,xe2x80x9d which contains some of the industry association standards for coloring concrete.
Inorganic pigments are typically used as color additives for building materials and typically include iron oxides (natural and synthetic), chromium oxide, cobalt blue, titanium dioxide and carbon black. However, these inorganic pigments offer a limited ranges of colors and brightness.
Organic pigments have not been used to color building materials as it is believed they lack sufficient alkali resistance and lightfastness. In 1981 the American Society for Testing and Materials (ASTM) in a report entitled xe2x80x9cPigments for Integrally Colored Concrete,xe2x80x9d discussed the test results of various inorganic and organic pigments for lightfastness, alkali resistance, water wettability and curing stability. All of the organic pigments tested, including phthalocyanine green, failed to meet the lightfastness testing standards.
Dry pigment powders have been used to color concrete compositions because they are highly dispersible. However, these powders have poor processing properties, and typically cake together and form lumps upon storage. They also tend to form dust.
The use of free flowing granules or heads to overcome the processing problems and dust associated with dry pigment powders has been suggested. These granules may be produced by spray drying aqueous dispersions, as proposed by U.S. Pat. Nos. 4,946,505; 5,484,481; 5,853,476; and 5,882,395. However, the evaporation of the aqueous dispersion requires expensive equipment and significant energy expenditures which can make the use of such granules economically unattractive.
Another approach is to modify the particle""s surface to improve its dispersibility in aqueous solution. U.S. Pat. No. 5,401,313 discloses a pigment particle whose surface is coated with an electric charge modifying agent and a dispersion promoting agent. The dispersion promoting agent is selected from stearates, acetates, alkylphenols, cellulosics, waxes, lignins, acrylics, epoxies, urethanes, ethylenes, styrenes, propylenes and polymers having functions groups of alcohols, glycols, aldehydes, amides and carboxylic acids, and is preferably sodium lignosulfonate for cementitious application systems. The surface-modified particle may be produced in powder, dispersion or granular form, with bead granules having a particle size of 25 to 250 microns being preferred.
An object of the invention is to provide aqueous organic pigment dispersions for coloring concrete and which exhibit acceptable alkali resistance and lightfastness.
The inventive dispersion is a combination of organic pigment, silica and water.
An advantage of the present invention is the ability to color building materials such as concrete with bright organic pigments that do not suffer from poor alkali resistance and/or poor lightfastness.
Another advantage of the present invention is that it permits the ready removal of graffiti or other surface defacement from a concrete surface without impairing its surface appearance.
In one aspect, the present invention relates to an aqueous based organic pigment dispersion, comprising
(i) silica binder;
(ii) organic pigment;
(iii) dispersing agent; and
(iv) water.
In another aspect, the present invention relates to a method for preparing organic pigment dispersions which includes
(i) mixing, at 3,000 to 10,000 rpm, a silica binder, organic pigment and dispersing agent, to form a dispersion premix;
(ii) milling the dispersion premix in a mixer filled with glass beads for a period of time sufficient to reduce the particle size of the organic pigment to from about 100 to about 300 nanometers, thereby forming a non-standardized dispersion;
(iii) adding water to the non-standardized dispersion until it matches a color standard and forms an organic dispersion.